Conventionally, a power storage apparatus has been known that includes an integrated circuit (a battery monitor IC) to detect voltages of unit batteries connected in series in a battery pack, and to monitor states of the respective unit batteries based on the voltages of the unit batteries detected by the battery monitor IC (see, for example, Japanese Laid-open Patent Publication No. 2013-162635).
FIGS. 1A-1B are diagrams that illustrate an example of a battery monitor apparatus that detects voltages of unit batteries included in a battery pack by using voltage detection ICs as in the conventional art (e.g., Japanese Laid-open Patent Publication No. 2013-162635). Specifically, the diagrams illustrate an example of a configuration to detect voltages of multiple unit batteries (battery cells C1 to C54) that are connected in series and included in a battery pack 100 constituted with multiple battery modules 100-1 and 100-2, by voltage detection ICs 200 (200-1 to 200-8).
Referring to FIG. 1A, each of the voltage detection ICs 200 detects voltages of eight battery cells that are connected in series, by voltage detection lines that are connected to nine ports (channels ch1 to ch8). That is to say, in each of the voltage detection IC 200, a voltage detection point (a voltage detection line), which corresponds to electrodes of two adjacent unit batteries connected with each other, is shared by two adjacent channels. This decreases the number of ports and the number of wires of a voltage detection IC, compared to a case where voltages are detected by connecting the respective electrodes of a unit battery with a voltage detection IC one by one. Therefore, the cost and the circuit size can be reduced.
On the other hand, when adopting such a configuration, if a battery pack is constituted with multiple battery modules connected by conductive members, voltages of some unit batteries (e.g., the voltage of the battery cell C28 in FIG. 1A) may be detected as voltages that include voltage drops caused by resistance of the conductive members. Specifically, if the number of unit batteries included in a battery module is not a multiple of the number of channels of the voltage detection IC, some voltage detection ICs may detect voltages of unit batteries over multiple battery modules. Therefore, at least one of detected voltages of unit batteries adjacent to a conductive member connecting two battery modules with each other, may include a voltage drop due to the resistance of the conductive member. Usually, since unit batteries in a battery module are placed closely together, their electrodes may be directly connected, or connected by connection members having very short length. Therefore, the resistance of such an “intra-module” connection member has little influence on the voltages of unit batteries, which are to be detected by the voltage detection IC. In contrast to this, a conductive member connecting battery modules with each other has a certain length due to a layout restriction and/or maintainability. Therefore, the resistance of such an “inter-module” conductive member has very large influence on the voltages of unit batteries, which are to be detected by the voltage detection IC. In this case, a voltage including a voltage drop due to the resistance of the conductive member connecting battery modules with each other is detected as the voltage of a unit battery, and hence, precision of the detected voltage of the unit battery may be reduced, and it may not be possible to monitor the state of the unit battery appropriately.
Therefore, as an example, as illustrated in FIG. 1B, the apparatus may be configured so that no voltage detection IC is provided that detects voltages of unit batteries over multiple battery modules. In this case, the influence of the resistance of an “inter-module” conductive member can be excluded, and a voltage detection IC can detect the voltage of a unit battery on the boundary of modules, with high precision.
However, if the apparatus is configured so that no voltage detection IC is provided that detects voltages of unit batteries over multiple battery modules, the number of voltage detection ICs needs to be increased as many as the number of “inter-module” conductive members. Especially, considering maintainability and the like, if a battery module is adopted that is constituted with a comparatively less number of unit batteries, the number of battery modules included in a battery pack may increase comparatively greater. In this case, the number of voltage detection ICs to be installed increases proportional to the number of battery modules. Therefore, the cost and circuit size may increase due to the increased number of voltage detection ICs to be installed.
In view of the above problem, it is an object of at least one embodiment to provide a battery monitor apparatus that can detect voltages of unit batteries in a battery pack having multiple battery modules connected in series where each battery module includes multiple unit batteries connected in series, and can monitor the unit batteries appropriately by the states of the respective unit batteries based on the detected voltages, without increasing the cost and circuit size of the apparatus.